Gas Planning 101: SAC/RMV and ways to make sure you have enough gas to complete your dive

(Part one of three-part lecture series, first delivered April 3, 2001

“Always plan ahead. It wasn’t raining when Noah built the ark.”
Richard James Cushing, 1895-1970, Roman Catholic Cardinal and Archbishop of Boston, MA

The reasons for bothering…
Noah would have had a couple of things going for him if he’d decided to become a technical diver. To begin with, he had two of everything, which is not a bad start down the road to contingency planning. Secondly, he is associated with water and lots of it. And finally, he was a guy who seems to have no worries making and following a plan while ignoring the jibes of the folks around him who couldn’t imagine forty days and nights of rain.
Planning is a wonderful habit to cultivate for anyone actively diving beyond traditional sport limits. Hold on… let me expand on that. Planning is a wonderful habit to cultivate for everyone diving beyond or within sport diving limits, but it is especially critical for anyone doing technical or advanced dives, because without a good solid dive plan, there cannot be a good, solid dive.
As we will discuss through the next several [chapters] there are abundant and assorted risks associated with advanced diving. A good solid dive plan helps to avoid or mitigate those risks and lies at the core of being a successful technical diver.
Among all the risks a diver has to account for, none is of greater consequence than gas management and no aspect of an advanced or technical dive plan is more essential than a concise, accurate gas management plan. This holds true for dives carried out on open-circuit scuba and on all flavors of rebreather – closed and semi-closed.
Nothing makes for a more stressful dive for an open circuit diver than running low on backgas or decompression gas, and nothing turns a rebreather diver’s hair grey faster than scrapping around the bottom of a diluent or oxygen cylinder for the last few litres of usable gas.

Completely running out of something suitable to breath, will totally ruin everyone’s groove – CCR diver or traditional open-circuit diver, regardless of how close a buddy or a bailout cylinder may be.
Astonishingly, although drawing up a gas management plan is fundamental and ranks as a primary-level skill, the basic constructs are often misunderstood, and this leads to some exceedingly dodgy dive plans.
The root of the problem could be a carryover from sport-diver practices. These  essentially boil down to: start with a fully charged dive cylinder, monitor your SPG, and surface – or arrive at the safety stop – with something between seeds and stems and a sixth of your starting pressure – the ubiquitous 500 psi for those familiar with dive briefings on Caribbean dive ops. If followed, all other things begin equal and the SPG having been serviced, recently calibrated and working correctly, this technique will likely get a diver home. But if anything goes pear-shaped during the dive, it provides an inadequate gas reserve and an unsuitable margin for error.
The classic OOA (Out Of Air) Emergency for a sport diver is usually the inevitable result of operator error: one diver gets excited or tense and burns through his gas supply much more rapidly than expected. There’s no gas plan and no reserve so the sudden shortage of something to breathe comes as a total surprise. Either a late glance at an SPG or a sensation similar to having an overweight Labrador retriever sitting on his chest, triggers a wide-eyed grab for the nearest functioning regulator. This is usually in the mouth of an unsuspecting “buddy” who has little forewarning of what’s about to happen, and no idea how to deal with it. The usual “next step” is two divers closing in on panic as they take a rapid flight to the surface accompanied by the savage screams of ascent alarms.
In the majority of cases, both divers make to the surface with little worse than a scare and an open invitation sometime in the future with aseptic necrosis. Occasionally though the outcome is far worse with an immediate payoff. Some victims of a runaway ascent, even when their dive was shallow and short, surface with DCS or lung over-expansion injury.
In advanced or technical diving, the gas planning process has to be more thorough and circumspect since running low on gas and rushing to the surface is a surefire guarantee of a visit to an emergency room, hyperbaric facility or morgue.
Technical divers therefore estimate gas usage within much tighter tolerances. There are plenty of different methods that work. Probably the best is to draw on personal data pulled from previous dives to similar depths and in similar conditions. But that assumes divers take notes, refer to them on a regular basis and are familiar with the process of adjusting things to suit shifting circumstances. These are the divers who can tell you how much gas they will have left on their back  at just about any point during their dive. Chances are you are not at that level yet, so let’s work through an example.
First a confession: cooking up a gas plan does take some effort and doing it for the first time can be about as much fun as de-worming the family cat. But really it is simple work with a non-scientific calculator (no trig, no functions, no fancy stuff, just ratios). The secret of making it as pain-free as possible is to understand the steps, follow them and know when to round up, approximate or “eye-ball” numbers. Of course, it’s all worth the effort because the benefits are boundless. You will learn a vital skill, create a mental template that can be reused again and again, and your gas plans will be enviously sublime.
When doing something new for the first time, I find it helpful to set myself some goals, so let’s look at the goals you should have for your first crack at creating a workable dive plan. Aim for moderately accurate and conservative, not perfection. Plan for a tolerable margin of error and work at compensating for that error – shaving it finer and finer – as you gather more and more actual data from more and more dives. Your final destination is being so tuned into your gas consumption during a dive that will you know what your Submersible Pressure Gauge (SPG) is going to read before you look at it! If you’re really good, you’ll know what your buddy’s says too.
What follows then is a step by step breakdown in both sensible metric and insane imperial. This assumes open circuit diving. We’ll deal with CCR gas planning separately, but my advice is to follow along even if you dive rebreathers.
To draw up an example plan we’ll work out some figures for a couple of open circuit divers: Vlada and Joseph.
Just to make it interesting, Vlada dives metric while her mate Joseph is American and thinks in US imperial. Luckily, Joe also dives a CCR on occasion and understands metric.
The first step for Joe and Vlada is to know what volume of gas they each consume in a minute at rest on the surface. Outfitted with this figure, everything else is simple arithmetic. And in the best traditions of high school mathematics, this figure needs to be a constant for future calculations to work.
Now here’s where the semantics start.
Diving is an easy-go-lucky kind of sport and many terms used by divers are elastic in their application. This drives scientific types and the geeky kids up the wall. It fazes me too. When working out gas volume requirements for a dive, I like to have one constant for each diver’s consumption rate on the surface. Once armed with this, all the other considerations such as depth, time, workload, temperature, narcosis, mental stress, fitness levels, how we feel today and what we had for breakfast can be factored in. The key piece of information is having a constant for surface air consumption in a state of rest.
It’s always seemed to me that Surface Air Consumption (SAC) is the perfect candidate. SAC is a unit measure of gas consumption on the surface, and since we need to have a constant non-variable figure to hang all the other factors from, SAC seems to win on several scores not least of which is its name.
And so, when I am planning gas volume requirements, I use SAC as a constant to describe an individual diver’s air consumption rate on the surface – and most importantly – at rest.  This does away with the need to use an array of potentially confusing terms such as average SAC, resting SAC, swimming SAC and so on.
If SAC is a non-variable figure – that’s to say, a person’s SAC does not vary from dive to dive – we need another term to describe what happens on a dive. On a dive, gas consumption rate  is influenced by a bunch of variables all present in different strengths and forms. I nominate RMV (Respiratory Minute Volume). RMV is the volume of air which can be inhaled (inhaled minute volume) or exhaled (exhaled minute volume) from a person’s lungs in one minute. RMV is a variable.
In common practice, many divers calculate how much gas they will need by working backwards from the volume of gas used on several logged dives. The standard method is to take the volume of gas consumed on each dive, divide it by bottom time and reduced that figure to a surface value by dividing by the depth expressed in atmospheres. The result is the amount of gas that they would have used each minute if their dive had been conducted on the surface. I have an issue with this from a detailed planning perspective, and feel the method yields inaccurate results because the consumption rate reflects how much gas is used while working… swimming, pushing dive gear through water, fighting current, and under various other variables like thermal stress, and narcosis.
In short, the figure arrived at using this method can’t accurately be used as since it’s invariably high. Typically at least half again over what true SAC would be. Not that this invalidates the method entirely. It’s just that with the weighting for all those variables already factored into what is supposed to be neutral number, it’s difficult to accurately forecast for widely different conditions.
Let’s return to our example. Vlada is new to technical diving and needs to work out her SAC rate, but she is not clear how to do it. Joe tells her to sit down at home watching TV and breathe from a small volume cylinder and keep track of time. Vlada sets up a 6 litre tank containing 125 bar of air and sets the timer on her stove for 30 minutes. She sits down, puts the regulator into her mouth and listens to the radio. After 30 minutes the timer buzzes and she notes that the SPG shows the remaining pressure is 70 bar. The calculation for her 30 minute consumption involves multiplying the pressure drop – 55 bar – by the cylinder capacity – 6 litres – and this gives 330 litres. She divides this by 30 to find out how much she breathes in one minute and arrives at 11 litres per minute. Vlada writes her SAC in her dive notes.
A couple of days later Joe and Vlada get together to create their gas plan for a dive the two intend to make the following Saturday afternoon.
Vlada and Joe plan to dive to 45 metres. At this depth, the ambient pressure will be about 5.5 atmospheres. Vlada arrives at this figure by moving the decimal point one place to the left to find the water pressure, and adding one to account for the surface air pressure (10 metres of water exerts about one atmosphere or which is close to one bar… one of those approximations it’s OK to make). So Vlada now knows that at 45 metres, the absolute pressure is about 5.5 atmospheres, and because of this she knows she will need 5.5 times the density of gas she would at the surface for each breath.
Armed with this information, she quickly works out how much air she would breathe in one minute at depth doing a similar activity to sitting in her kitchen listening to the radio: 11 X 5.5 litres or 60.5 litres.
Having gotten this far with the constant values, she and Joe start to plug in the variables for her estimated RMV. The variables measure estimates for increased breathing because of physical stressors such as workload, current, temperature, and the amount of gear she’ll be pushing through the water. She also needs to make some allowance for increased gas consumption because of mental stress caused by things like poor visibility, narcotic loading and diving in an unfamiliar spot. All these variables combined are known as the Dive Factor or DF.
The base standard DF for a dive in familiar waters with little workload and the minimum of stressors is 1.5. For example, if Vlada’s dive were to fall into this category she would multiply her 60.5 litres per minute by 1.5 to factor in the dive factor: 90.75 litres per minute. But the planned dive with Joe will be in unfamiliar cool water, carrying a stage bottle of decompression gas. Joe suggests a DF of 2 for her. This translates into a volume of 121 litres per minute for Vlada’s dive. This is Vlada’s Respiratory Minute Volume (RMV) for this dive and will form the central strut of her gas management plan.

(to be continued…)